In many television broadcast signals, notably those adhering to the NTSC and PAL standards, there is a defined relationship between the horizontal line frequency (Fh) and the color subcarrier frequency (Fsc). For example, in NTSC signals, luminance and chrominance are transmitted in a single channel by using a chrominance subcarrier at 3.58 MHz, which is equal to the line frequency, 15,734.26 Hz, times 455/2. This relationship makes it possible to separate luminance and chrominance components in the television receiver, thus avoiding interference between them prior to the recovery of the primary color signals for display.
In digital television systems that follow NTSC standards, typical digital color separation algorithms call for a sampling rate that is some multiple of the 3.58 MHz subcarrier frequency. A commonly used sampling rate is approximately 14.318 MHz, referred to as the "4 fsc" rate. It results in approximately 763 samples of active video data per line.
A problem with the 4 fsc sampling rate is that it is not always consistent with the sampling rate required for a desired number of pixels per line (horizontal resolution). For example, today's enhanced definition television systems, such as "wide NTSC", call for an image that is wider than previous images, with an aspect ratio of 16:9. The desired aspect ratio and the available number of lines per frame determine the horizontal resolution.
The simplest sampling methods provide one sample per pixel. But, in general, the sampling rates for NTSC color separation and for a desired aspect ratio are not the same. For example, for a 480 line display with a 4:3 aspect ratio 640 pixels per line are required. However, as stated above, the 4 fsc sampling rate for NTSC color separation results in 763 samples per line. This requires some sort of means for providing samples for the desired horizontal resolution.
Some existing systems provide the required number of pixels per line by first sampling the incoming data at 4 fsc or some other color burst-related sampling rate, performing color separation, and then scaling the samples to the desired horizontal resolution. However, a problem with scaling is that visual artifacts are likely to occur. Also, the increased complexity of such systems makes them more costly.